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A 2H and 13C NMR multifield relaxation study of some nonaqueous systems of sodium dodecyl sulfate
A 2H and laG NMR Multifield Relaxation Study of Some Nonaqueous Systems of Sodium Dodecyl Sulfate A. C E G L I E , *
M. MONDUZZI,'~'z~ AND O. S O D E R M A N §
* Dipartimento di Chimica, Universitd di Bari, 70126 Bari, Italy; tDipartimento di Scienze Chimiche, Universitd di Cagliari, 09124 Cagliari, Italy, and §Physical Chemistry 1, Chemical Centre of Lund, S-221 O0 Lund, Sweden
Received October 27, 1989; revised July 2, 1990 A multifield 2H and ~3C NMR relaxation study was carried out on some two-, three-, and fourcomponent systems of sodium dodecyl sulphate in nonaqueous solvents such as formamide, N-methyl formamide, or N,N-dimethyl formamide. Pentanol or octanol was used as cosurfactant, while p-xylene was added to obtain nonaqueous microemulsions. Relaxation data were mainly analyzed in terms of a "two-step" motional model in order t o obtain details on the structure and dynamics of the systems. The analysis of the data demonstrated some peculiar features of these nonaqueous systems, and analogies and differences with respect to the corresponding aqueous systems were investigated. It was shown that the degree of organization of SDS in the nonaqueous solvents is generally rather low. Among the results the effect of the addition ofoctanol as cosurfactant to the two-component systems should be mentioned. In the case of formamide systems, this alcohol seems to induce the formation of an organized solution, although the occurrence of any closed domain cannot be proved. A totally different behavior is observed in N-methyl formamide and N,N-dimethyl formamide. In these solvents, octanol seems to act as a structure breaker, thus leading to largely structureless solutions. © 1991AcademicPress,Inc. INTRODUCTION S u r f a c t a n t aggregates in a q u e o u s s o l u t i o n have b e e n widely investigated b y m e a n s o f m a n y different techniques. In the last d e c a d e the interest has also focused on the s t u d y o f surfactant self-association in n o n a q u e o u s syst e m s a l t h o u g h aggregation p h e n o m e n a o f s u r factants in s o m e organic p o l a r solvents have been p r e v i o u s l y o b s e r v e d ( 1- 13 ). M o r e o v e r , n o n a q u e o u s m i c r o e m u l s i o n s have b e e n used as r e a c t i o n m e d i a ( 1 4 ) . These facts h e l p e d to rise the interest in a n d the d e m a n d for a deeper k n o w l e d g e o f these n o n a q u e o u s systems. A t present, a l t h o u g h the phase d i a g r a m s o f several n o n a q u e o u s systems have b e e n determ i n e d , very little is k n o w n c o n c e r n i n g m i c r o structure, shape, a n d size o f the aggregates a n d flexibility o f the interfaces. C o n t r a d i c t o r y results have been o b t a i n e d b y different experim e n t a l t e c h n i q u e s even o n the m o s t investiTo whom correspondence should be addressed.
gated systems. F o r instance, the o c c u r r e n c e o f micellar aggregates o f s o d i u m dodecyl sulphate ( S D S ) in f o r m a m i d e ( F M ) was h y p o t h e s i z e d in Ref. ( 6 ) on the basis o f c o n d u c t a n c e m e a surements. L a t e r it was shown t h a t s o m e k i n d o f " s o l u t e - s o l v e n t particles" i n s t e a d o f true micelles o c c u r in this system ( 15 ). H o w e v e r , the o c c u r r e n c e o f SDS micelles in F M was indicated at higher t e m p e r a t u r e s ( T a b o v e 5 5 °C) ( 1 6 ) . Likewise in the c e t y l t r i m e t h y l a m m o n i u m b r o m i d e - F M system the o c c u r r e n c e o f spherical micelles at 6 0 ° C was a s c e r t a i n e d b y m e a n s o f a 1H, ~3C, a n d 14N multifield N M R r e l a x a t i o n study ( 1 7 ) . R e c e n t l y a r a t h e r extensive N M R self-diffusion s t u d y ( 1 8 ) , carried o u t at r o o m t e m p e r a t u r e o n s o m e two-, three-, a n d f o u r - c o m p o n e n t SDS systems with F M , N - m e t h y l formamide (NMF), and N, N-dimethyl forma m i d e ( D M F ) as solvents, d i d not indicate a n y appreciable confinement of any component in closed d o m a i n s , or the presence o f interfaces
129 0021-9797/91 $3.00 Journal of Colloid and Interface Science, Vol. 142, No. 1, March 1, 1991
Copyright © 1991 by Academic Press, [nc. All rights of reproduction in any form reserved.
130
CEGLIE, MONDUZZI,
as found for the aqueous systems. Some preliminary 2H relaxation data (19) showed that SDS does not give any appreciable micellization in FM at 26°C, in agreement with selfdiffusion data ( 18 ) and previous findings ( 15, 16). However, the addition of octanol to this system seems to induce some kind of organization. The same trend is found in the fourcomponent S D S - F M - o c t a n o l - p - x y l e n e microemulsion; however, relaxation data were not sufficient to gather details on molecular dynamics. Therefore we have undertaken a systematic study of these nonaqueous systems by means of 2H and 13C N M R relaxation to clarify some aspects of the aggregation phenomena, if any, at room temperature. Multifield 2H and 13C relaxation studies have been extensively used to investigate molecular motions in surfactant solutions (2026). In many cases the rationalization of the spin-lattice and spin-spin relaxation data in micellar surfactant-water systems was made by the use of a motional model, the so-called "two-step model" (23), on the basis of which one considers two kinds of motions: a fast local motion, slightly anisotropic, due to the reorientation of the methylene groups within the alkyl chain of the surfactant, and a slower motion, isotropic and superimposed on the former, due to the tumbling of the whole aggregate a n d / o r to the diffusion of the surfactant monomers over the aggregate surface. This model was proved to apply also in the case of several aqueous microemulsions (20, 27). From the analysis of N M R relaxation data we want to draw a profile of structure and dynamics of these waterless systems in order to identify analogies with and differences from the corresponding aqueous systems (19, 28, 29). THEORETICAL
BACKGROUND
The 13C spin-lattice relaxation rates of methyl and methylene carbons are mainly due to the dipole-dipole interaction between carJournal of Colloid and Interface Science, Vol. 142, No. 1, March 1, 1991
AND SODERMAN
bons and their directly bound protons. The expression for 13C R 1 is given by (30)
L 4S r . + 3J(wc) + 6J(wH + wc)l.
[ll
For a nucleus with spin I = 1, like deute-
rium, the spin-lattice (R1) and spin-spin (R2) relaxation rates are (30) 37r 2 R, = ~ - - X2[2J(wD) + 8J(2c0v)]
[2]
37r2 R2 = ~ x213J(0) + 5J(WD) + 2J(2wD)]. [31 In these equations Ni is the number of protons directly bound to the carbon; ~zo is the permeability of vacuum; ~/r~ and 7c are the magnetogyric ratios of proton and carbon; rcn is the C - H bond length (set equal to 1.09 A, the value normally used for a sp3-hybridized carbon: it has been suggested that a value of 1.11 A ( 31 ) should be used; however, negligible differences are obtained for the parameters calculated with these two values of rcH); J(w) are the various reduced spectral density functions; and Wc, wn, and wD are the Larmor frequencies of carbon, proton, and deuterium at a given magnetic field strength. In Eqs. [ 2 ] and [3], x is the quadrupolar coupling constant. Please note that the asymmetry parameter of the electric field gradient tensor for the C - D group is set equal to zero (32). By introducing in the rationalization of the relaxation data the two-step model, which assumes a slow isotropic motion superimposed on a fast, slightly anisotropic, motion and by assuming that the "extreme narrowing conditions" apply to the fast motion and furthermore that the slow motion can be described by a single exponential correlation function, we can write the reduced spectral density function as (33, 34) J(w) = ( 1 -- S2)2rcr+ S 2
2r~
1 + (oOr~c) 2 "
[4]
NMR OF SDS IN NONAQUEOUS SOLUTIONS S is the order parameter defined as S = (½)(3 cos2~9 1 ) f , where the average is taken over a time that is long enough to average the anisotropic fast local motion but much shorter than the time characteristic of the slow motion. 0 represents the angle between the C - H or C - D vector and the local director, taken to be normal to the surface of the aggregate. ~-~ and r~ are the correlation times for the fast and slow motions, respectively. -
-
EXPERIMENTAL Materials. Sodium dodecyl sulphate (99%,
biochemical grade), 1-pentanol (98 % ) (PEN), 1-octanol ( 9 9 % ) ( O C T ) , formamide (analaR), N-methyl formamide (analaR), N,N-dimethyl formamide (analaR), and pxylene (98.5%) ( X Y L ) were all from BDH, England. All the reagents were used as received without further purification, a,a-2H2-SDS (DSDS) was prepared following a procedure previously described (29). Methods. N M R 13C relaxation measurements were run at 1.4, 2.35, 4.7, and 8.47 T on a Jeol-FX-60, on a Bruker MSL 100, on a Varian XL-200, and on a Nicolet NTC-360 spectrometer, respectively. 2H N M R relaxation measurements were performed at 4.7 T on the Varian XL-200 spectrometer. In the 13C experiments ~H decoupling was carried out by means of square wave modulation of the decoupler carrier centered in the proton frequency range. At the highest magnetic field strength a bilevel decoupling sequence was used in order to avoid overheating phenomena. The temperature in all measurements was 26.0 + 0.5°C. The partially relaxed spectra were obtained by means of the usual "inversion recovery" sequence. The spin-lattice relaxation rates were calculated using a three-parameter nonlinear fitting to 14-20 experimental points. The spin-spin relaxation rates for 2H were deduced from the bandwidths of the 2H N M R signals taken at half-height after suitable correction for the magnetic field inhomogeneity. Computational details. With reference to
131
the two-step model we assumed that the acarbon of the surfactant was the most sensitive to the slow motion as it is closest to the nonpolar-polar interface. Then we chose a value ofr~ and from the experimental 13C Rl values of the a-carbon obtained at two different magnetic field strengths we calculated analytically the values of r f and S. These values of r~c, z~r, and S were used to calculate the relation 2HR1 Rcalc = 2HR2 2J(WD) + 8J(2wD) = 3 ) ( 0 ) + 5](~D) + 2J(2~0D),
[5]
which is the ratio between Eqs. [2] and [3] after the parameter X2 is eliminated. The function J(a~) is given by Eq. [4]. The value of Rc,lc so obtained was compared with that obtained from the experimental ratio Robs = [R1/R2] for 2H. The value ofr~ which minimizes the quantity ERR =
(Robs - RcaIc) 2 (Robs) 2
[6]
was then chosen as the best value ofr~ for the whole aggregate, r f and S values for all resolved carbons of the surfactant were then calculated. When 13C R1 values at more than two magnetic field strengths were available, these other data were included in the minimization procedure and the minimized quantity becomes ERRa-= ERR + ~
(Rlobs
-- Rlcalc)
~b~
2
, [7]
K,I
where K and I indicate the magnetic field strength and the carbon, respectively. RESULTS AND DISCUSSION In Table I we report the 2H relaxation data together with the composition (wt%) of all the examined samples. In the same table the 2H relaxation rates obtained at 6.0 and 8.47 T for some three- and four-component systems with FM (from Ref. (19)) are also reported while the 2H data obtained for the corresponding Journal of Colloid and Interface Science, Vol. 142, No. 1, March 1, 1991
132
CEGLIE, MONDUZZI, AND SODERMAN TABLE I 2H Relaxation Data at 26°C for Samples Containing DSDS 4.7 T
Systemand composition(wt%) SDS-FM (1-99) (4-96) (6.5-93.5) SDS-FM-alcohol (15-55-30) PEN OCT SDS-FM-alcohoI-XYL (16-40-32-12) PEN OCT SDS-NMF (2-98) (8-92) 15-85) SDS-NMF-alcohol (15-55-30) PEN OCT SDS-NMF-alcohoI-XYL (16-40-32-12) PEN OCT SDS-DMF (2-98) (5-95) (8-92) SDS-DMF-alcohol (11-67-22) PEN OCT SDS-D20 d (5-95) (10-90) (20-80) SDS-H20-PEN a (20-43.6-36.4) SDS-H20-alcohol-XYL a (17.5-35-35-12.5) PEN OCT
RIa (s-I)
R2b(s-~)
16.7 +_ 0.3 18.3 +_ 0.1 20.6 +_ 0.2
19.0 20.7 24.7
0.88 +_ 0.07 0.88 +_ 0.07 0.83 +_ 0.05
36.1 +_ 0.3 37.9 +_ 0.3
48.5 79.4
0.74 +_ 0.02 0.48 +_ 0.01
36.0 +_ 0.3 39,5 +_ 1.1
49.8 87.0
0.72 +_ 0.02 0.42 +_ 0.01
11.1 +_ 0.3 12.3 _+ 0.1 15.6 +_ 0.1
14.6 16.0 19.0
0.76 +_ 0.08 0.75 +_ 0.08 0.82 _+ 0.07
19.5 _+ 0.1 24.5 +_ 0.2
23.3 25.4
0.84 +_ 0.06 0.96 +_ 0,06
23.2 +_ 0.2 28.7 +- 0.3
24.1 28.8
0.96 + 0,06 0.99 +_ 0,06
6.9 +_ 0.1 7.8 +_ 0.1 8.8 +_ 0.1
9.1 10.1 10.7
0.76 +_ 0,13 0.77 +_ 0.12 0.82 +_ 0.12
12.8 +_ 0.1 14.0 +_ 0.1
17.3 16.0
0.74 +_ 0.07 0.87 _+ 0.09
28.4 +_ 0.7 29.2 _+ 0.2 31.0 +- 0.2
42.9 48.0 51.0
0.66 +_ 0.03 0.61 _+ 0.02 0.61 +- 0.02
26.0 +_ 0.2
56.8
0.46 +_ 0.01
27.0 +_ 0.1 26.2 +_ 0.2
54.6 182.8
Ro~C
0.49 +_ 0.01 0.14 +_ 0.002
6T
SDS-FM-OCT f (15-55-30) (10-40-50) SDS-FM-OCT-XYL y (13,3-30-26.8-29.9) (13,4-45-26.7-14.9)
8.47 T
R~~(s-')
Rf (s-~)
Ro~~
R~~(s~)
32.9 +_ 0.9 32.2 +_ 1.0
62.5 +_ 3.9 55.6 +_ 6.2
0.53 +_ 0.04 0.58 +_ 0.07
27.9 +_ 0.2 27.2 +_ 0.4
29.9 +_ 0.6 26.3 _+ 0.7
79.4 +_ 5.0 66.7 _+ 4.4
0.38 +_ 0.03 0.39 _+ 0.03
26.1 +_ 0.2 25.6 + 0.2
a The reported errors in R1 represent the error of the three-parameter nonlinear fitting. b R2 data are obtained from 2H N M R bandwidth taken at half-height in a spectrum recorded with a pulse angle of 90 °. A Lorentzian line fitting of the experimental data points confirmed the Lorentzian shape of the 2H NMR signals to within an error of +_0.5 Hz, which corresponds to an error of _+).6 s-1 in R2. The correction for the inhomogeneity contribution was made on the basis of the values of Rl and R2 of a D20 reference sample measured before and after recording each sample. c The error in R2 corresponds to the propagated error resulting from the errors in R1 and R2. d From Ref. (29). e From Carr-Purcell-Meiboom-GiU experiments. f F r o m Ref. (19). Journal of Colloid and lnterface Science, Vol. 142,No. 1, March 1, 1991
NMR OF SDS IN NONAQUEOUS SOLUTIONS 136R1
t
SDS-
S-1
7 -93
2.0
,i
SDS-NMF
FM
15
%
-
85
,) ~ ,~ (, ,b (,
,)
,)
o
Z
:
#
-{
####~Q
#l
# ~01 A i ~A| #* •
hO
I
|
! !
123456789101112
1 23456789101112
SDS
carbons
FIG. I. Experimental ~3CR~ for the carbons of SDS in the two-component systems. Frequency: (O) 1.4 T, (11) 4.7 T, (A) 8.47 T. aqueous systems are given for comparison (from Ref. (29)). The ~3C spin-lattice relaxation rates R~ are shown in Fig. 1 for the twoc o m p o n e n t systems, in Fig. 2 for the threec o m p o n e n t systems, and in Fig. 3 for the fourc o m p o n e n t microemulsions with pentanol as cosurfactant.
~3C R
Some considerations arise from a first observation of the experimental results: (a) In the micellar S D S - H 2 0 system the 2H R~ and R2 values and, in particular, their ratios, Robs'S, were found to be almost constant with increasing SDS concentration above CMC. This fact is indicative of the occurrence of an almost constant ratio between micellized and free surfactant molecules. The values obtained for Robs above C M C are around 0.6, while below CMC the value is almost equal to unity (Robs = 0.93 _+ 0.15). Thus a value of Rob s significantly less than unity is indicative of the presence of surfactant aggregation. In the nonaqueous systems the values of Robs are around 0.8, thus indicating that the surfactant molecules occur to a large extent as monomers. A m o n g the three- and four-component systems, only the FM systems containing octanol as cosurfactant show Rob s values significantly lower than unity. In the aqueous threeand four-component systems the Robs ratios are always lower than 0.6, in particular, in the octanol microemulsion, where a value of 0.14 was found.
SDS-FM-PEN
S-1
15
- 55
- 30
133
SDS-NMF-PEN %
15
-
55
- 30
%
2.0
T A I
I
I
1.0
Aeee
•
1
¢
1
•JA!
[] •
SDS 2 34
56
El •
[]
•
!
• D
PEN 7 891011
1 2 3 4
SDS 234
5 6 7 8 91011
D D
PEN 1 2 34
carbons
FIG. 2. Experimental ~3C R~ for the carbons of SDS and PEN in the three-component systems with pentanol as cosurfactant. Frequency: ([]) 2.35 T, (i) 4.7 T, (A) 8.47 T. C~2signal of SDS is obscured by C5 of PEN. Journal of Colloid and Interface Science, Vol, 142, No. 1, March 1, 1991
134
CEGLIE, MONDUZZI, AND SODERMAN
13C Rlt
SDS- FM- PEN- p-XY L 16-40-
S D S - N M F - PEN-p-XYL
12
-
16
-
40-
32
-
12
t
2.0
1.0
32
+
":::t
t
• 1
Iiti 1
!
D
[]
"A~
| ~DS
PEN
1 234567
891011
PEN
SDS
I 234567891011
1 234
1 234
carbons FIG. 3. Experimental ~3C R~ for the carbons of SDS and PEN in the four-component systems with pentanol as cosurfactant and p-xylene as oil. Frequency: ([3) 2.35 T, (am)4.7 T, (A) 8.47 T. C~z signal of SDS is obscured by C5 of PEN and by the -CH3 groups of XYL
( b ) I n Figs. 1, 2, a n d 3 it can be seen t h a t t h e f r e q u e n c y d e p e n d e n c e o f the 13C R~'s in the various systems is generally larger for the a - c a r b o n o f SDS t h a n for the o t h e r carbons. T h i s is p a r t i c u l a r l y e v i d e n t in Figs. 4 A - 4 C ,
A
w h e r e the ratios b e t w e e n the 13C R~'s o b t a i n e d at t w o different frequencies for each c a r b o n in the v a r i o u s systems are reported. In the s a m e figure we also report, for c o m p a r i s o n , the ratios o b t a i n e d in t h e c o r r e s p o n d i n g a q u e o u s sys-
13 2 . 3 5 n 1
B
Rp-47
R~138 R~.47
~¢r
¢r ¢r ¢r ¢r'~'~ •
•
SDS
123456789101112
SDS
1234567891011
C
rosa
m
~
DO~DaQIImI!
~
PEN
SDS
PEN
1234
1234587891011
1234
carbons FIG. 4. Ratio between 13C R 1 'S for each carbon of SDS at two different frequencies: (A) SDS in the twocomponent systems, (R~h.4/(Rl)8.47; (B) SDS and PEN in the three-component systems, (RI)2.35/(R1)8.47; (C) SDS and PEN in the four-component systems, (Rx)2.35/(Rt)8.47. 13Cdata for the aqueous systems is from Ref. (29). (~) Aqueous systems, (11) FM systems, ([]) NMF systems. Journal of Colloid and Interface Science, Vol. 142, No. 1, March 1, 1991
135
NMR OF SDS IN NONAQUEOUS SOLUTIONS
marked frequency dependence of 13CR l's only in the aqueous systems (Figs. 4B and 4C). On the basis of these considerations it is evident that important differences between aqueous and nonaqueous systems occur in the molecular organization and in the dynamic behavior. In the analysis of the experimental data it must be taken into account that FM, NMF, and DMF differ from water in many important physical properties such as viscosity and dielectric constant (see Table II). However, since a frequency dependence of 13C R~ is observed and 2H R1 differs from 2H Rz in several cases, at first we made use of the twostep model approximation. Following the procedure described under s r~, and S for Experimental we calculated rc, the various systems. These parameters are reported together with the values of ERR (Eq. [ 6 ] ) and ERRT (Eq. [ 7 ]) in Tables III and IV.
TABLE 1I Viscosities 0/) and Dielectric Constants (~) of the Solvents at 25°C Solvent
~/(cP)
~a
H20 FM NMF DMF
0.891 3.30 1.65 0.796
78.8 109.5 182.4 36.7
a From the "Handbook of Chemistry and Physics," CRC Press, Cleveland, OH, 1983-1984.
tems (from Ref. (29)). The trend of these ratios indicates that the frequency dependence of the 13C R~'s in the nonaqueous two-component systems is significantly lower than that observed in the SDS-water micellar system (Fig. 4A). The same trend, although much less pronounced, is observed in the four-component systems (Fig. 4C), while no significant differences from the corresponding aqueous system are observed in the three-component systems with pentanol as cosurfactant (Fig. 4B). The a-carbon of pentanol shows a
FM Systems In the case of the two-component system, the experimental ~3CRl'S of the SDS a-carbon are very well reproduced by the values of r g
TABLE III FM Systems SDS-FM z~ = 2.5 ns
SDS-FM-PEN r~ = 7.0 ns
~
SDS-FM-PEN-XYL ¢~ = 7.0 ns
~
T~r
Carbon
(ps)
S
(ps)
S
(ps)
S
1 2 3 4 5 6 7 8 9 10 11 12
18.1 32.4 28.9 22.7 31.0 29.3 26.2 29.3 22.7 14.1 10.7 4.3
0.141 0.094 0.130 0.124 0.099 0.104 0.114 0.104 0.124 0.085 0.072 0.037
25.2 34.0 35.5 21.1 32.3 27.8 27.8 27.8 21.1 14.0 9.8 .
0.127 0.082 0.121 0.135 0. l 17 0.133 0.133 0.133 0.135 0.059 0.043
26.1 32.6 29.7 32.6 24.0 24.8 24.8 24.0 19.7 11.3 8.1
0.109 0.051 0.134 0.051 0.126 0.123 0.123 0.126 0.084 0.051 0.043
ERR ERR~
0.80 × 10-3 0.160
. 0.75 × 10-~ 0.73
.
. 0.33 × 10-~ 0.289
N o t e . r~, r~, and S parameters were calculated with the two-step model of approximation for SDS carbons. Journal of Colloid and Interface Science. Vol. I42, No. 1, March 1, 1991
136
CEGLIE, MONDUZZI, AND SODERMAN TABLE IV NMF Systems SDS-NMF r~ = 2.5 ns
Carbon
1
2 3 4 5 6-8 9 10 11 12 ERR ERRT
SDS-NMF-PEN r~ = 5.4 ns
SDS-NMF-PEN-XYL r~ = 6,0 ns
r re (ps)
S
(ps)
S
(ps)
S
16.8 16.2 19.9 16.2 18.1 16.2 16.2 8.1 7.2 3.8
0.111 0.091 0.102 0.091 0.084 0.091 01091 0.064 0.023 0.016
19.0 16.2 25.9 17.3 21.1 21.1 17.3 10.3 7.3
0.111 0.130 0.087 0.091 0.089 0.089 0.091 0.051 0.030
18.8 30.2 28.6 20.2 22.0 22.0 20.2 9.2 7.0
0.130 0.026 0.097 0.076 0.089 0.089 0.076 0.064 0.034
0.1 × 10 3 0.17 X 10-1
0.72 × 10-1 0.40
0.190 1.45
Note. T~, r~, and S parameters were calculated with the two-step model of approximation for SDS carbons.
= 2.5 ns, r~ = 18.1 ps, a n d S = 0.141 reported observed 13C R1 and 2H R1 and R2 relaxation in Table III, ~whereas 2H relaxation rates data. In fact motions should be rather complex slightly higher than the experimental values since they arise from rotations around carbonare calculated when X = 185 kHz (29) is in- carbon bonds, torsions, and rotations of the troduced in Eqs. [2 ] and [3 ]. A slow-motion whole molecule around its long axis. Moreover correlation time rg = 2.5 ns seems to be an motions exceeding the extreme narrowing unreliable value for interpreting the micellar limit can be expected either in the case where tumbling in a solvent such as FM. This solvent only SDS monomers occur in a viscous solvent has a viscosity which is three times higher than such as FM or in the case where small SDS that of water at room temperature; therefore, dimers, trimers, etc., a n d / o r some kind of slow-motion correlation times of ca. 10-8 s are molecular solute-solvent complexes are presexpected if SDS micelles of the same size as ent. It should be noted that the occurrence of those found in aqueous solution were present a molecular complex could account for the in FM solution. Moreover, the observation observed conductance variation (6) and could that 2H R1 depends rather strongly on the con- also be in agreement with the observation that centration of SDS implies that a substantial the self-diffusion coefficient of FM increases fraction of SDS molecules is present as mono- while that of SDS is almost constant as the mers. These results are in agreement with the : FM concentration increases i n the tv~o~com-. conclusions d r a w n self-d~ffusiofi,;mea~; " P o n e i a f s D S ' F M gygtem (i8i:. ' ~ ...... In conclusion, although in this case the twosurements and with our preliminary relaxation data (18, 19) as well as with the results ob- step model should not appropriately describe tained by fluorescence quenching in (15), the motional behavior of SDS molecules in which indicate the occurrence of a molecular FM solution, it can be considered as a qualitative description of the local motion (cf. r f) dispersed solution in the SDS-FM system. Although in this system no well-defined ag- and the degree of freedom (cf. S parameter) gregation form occurs, a single isotropic cor- of the various carbons and of the tumbling of relation time is inadequate for describing the the whole SDS molecules in FM (cf. ~_s). Journal of Colloid and Interface Science, Vol. 142, No. 1, M a r c h 1, 1991
N M R OF SDS IN N O N A Q U E O U S SOLUTIONS
In the three- and four-component FM systems with pentanol as cosurfactant we observe a marked increase of the spin-lattice and especially of the spin-spin 2H relaxation rates and consequently a decrease of the ratio 2H R~/R2 (see Table I). The analysis of the ~3C and 2H relaxation data in terms of the twostep model gave the results reported in Table III. Generally the ~3C R~ data of the a-carbon can be fairly well reproduced by these parameters but they cannot reproduce the 2H R~ and R2 values when X = 185 kHz is used in the calculations. As for the three-component system S D S F M - O C T , unfortunately the overlapping of most of the ~3C N M R signals of SDS with those ofoctanol prevented us from performing a detailed relaxation analysis. However, in this case, 2H relaxation data for the a-carbon of SDS are available at three different frequencies (19). It should be noted that the 2H R~ for the S D S - F M - O C T system does not depend on SDS concentration at 6.0 and 8.47 T (see Table I). Through the use of the two-step model the frequency dependence of the 2H relaxation rates can be accounted for by the values r~ = 6 ns, rcf = 35 ps, and S = 0.18, assuming X = 185 kHz (29). The results are shown in Fig. 5, where it can be seen that a fairly good agreement between the calculated and experimental 2H R~ and R2 values is obtained. In the case of the microemulsion with oc-
137
tanol, a further decrease of the ratio 2H R~/ R 2 is observed; however, the available experimental data give evidence of a dynamic behavior very like that found in the S D S - F M O C T system. The addition of the oil seems to induce a slight decrease in the rate of the slow motions. From self-diffusion coefficients of FM in these systems it can be inferred that FM is not extensively confined into closed domains. In particular, it should be mentioned that in the corresponding aqueous microemulsions the relaxation and self-diffusion data indicate the occurrence of a bicontinuous structure (Robs = 0.49) when pentanol is used as cosurfactant and of a water-in-oil droplet structure (Robs = 0.14) for the microemulsion with octanol as cosurfactant ( 18, 29, 35, 36 ). However, the fact that FM occurs in a continuous domain does not exclude the possibility that the SDS molecules are organized in some sort of aggregate, as shown by the 2H relaxation data. It should be noted that self-diffusion coefficients are significantly affected by the presence of a small fraction of nonassociated molecules, since Dobs = pDmon + ( 1 - p)Dagg (p is the fraction of molecules present as m o n o mers while 1 - p is the fraction of aggregated molecules) and Dmon ~ Dagg. On the other hand, relaxation measurements are significantly influenced by the presence of a small a m o u n t of aggregated molecules, since in this case Rlobs = pRlmoo + ( 1 - P)Rlagg and R1agg Rlmon.
S-1 2H R2
S D S - FM - O C T
lOO
50
Iog~ ?.5
8.0
FIG. 5. S D S - F M - O C T
8.5
9.0
9.5
In these systems the relaxation measurements, interpreted in terms of the two-step model, revealed significant differences arising from the use of different alcohols: octanol, unlike pentanol, seems to induce the formation of some kind of well-defined interface. In support of this the goodness of the results obtained in the case of the S D S - F M - O C T system by the use of the two-step model should be noted.
( 1 5 - 5 5 - 3 0 wt%) (O) 2H
N M F Systems (R l) expand (•) 2H (R2)expexperimental relaxation rates. Continuous lines: two-step model ZH (RI)~lc and (Rz)calc The experimental data for the two-comas a function oflog(oa)with X = 185 kHz, r~ = 6 ns, ~-~ = 35 ps, S= 0.18. ponent systems with N M F as solvent seem to Journal of Colloid and Interface Science, Vol. 142,No. 1, March i, 1991
138
CEGLIE, MONDUZZI, AND SODERMAN
indicate the occurrence of a structure and of a dynamic behavior rather similar to those found in the FM systems. On the contrary the 2H data, reported in Table I, show that the addition of the third and the fourth component induces an increase of the ratio 2H R l / R2 either in the case of pentanol or in the case of octanol. What is interesting is the observation that the use of octanol as cosurfactant seems to produce almost molecular-disperse solutions (cf. Table I, R1 = R2). The use of the two-step model led to the estimates of the parameters reported in Table IV. The set of parameters calculated for the two-component system can reproduce either the ~3C or the 2H relaxation rates rather accurately. Worse agreement is obtained in the case of the three- and four-component systems. The same considerations as those reported above for the FM systems, except those containing octanol, can be extended to N M F systems. In conclusion the N M R relaxation data seem to confirm the absence of any aggregated form of the surfactant molecules in N M F systems.
the corresponding aqueous systems have been displayed. One of the most interesting features is the effect of the addition of octanol to the systems with N M F and DMF. In these solvents, octanol seems to induce a complete breaking up of every kind of aggregation, unlike what has been observed in FM, where the occurrence of some sort of organized interface has been ascertained. The use of the two-step model is strictly correct only if some kind of organization is proved to occur to a certain extent, while in other cases the method represents only a qualitative approach to interpret molecular dynamics. Finally it should be noted that the analysis of the 2H relaxation rates seems to be very useful in discriminating between organized and molecular-dispersed solutions. ACKNOWLEDGMENTS
We are gratefulfor helpful discussionswith Bjorn Lindman. Financial support by the Swedish National Science Research Council, by the Swedish Board of Technical Development, and by the Italian National Council of Research (CNR), Comitato Tecnologico,is acknowledged. REFERENCES
DMF Systems
1. Mittal, K. L., and Fendler, E. J. (Eds.), in "Solution Behaviour of Surfactants," Vol. 2, part III, pp. 743-
The use of D M F as solvent should yield very different systems since both viscosity and dielectric constant are m u c h smaller than those of water, FM, and NMF. Only preliminary data are available for these systems; however, the 2H data, reported in Table I, suggest that the structure and the dynamic behavior should be very m u c h like those found in the N M F systems. Moreover, it should be pointed out that the further decrease of 2H relaxation rates in the D M F systems is clearly related to the decrease of the viscosity of this solvent as compared to that of F M and NMF.
886. 2. Reinsborough,V. C., and Bloom, H., Aust. Y. Chem. 20, 2583 (1967). 3. Reinsborough,V. C., and Bloom, H., Aust. J. Chem. 21, 1525 (1968). 4. Reinsborough,V. C., and Bloom, H., Aust. J. Chem. 22, 519 (1969). 5. Ionescu,L. G., and Fung, D. S., J. Chem. Soe. Faraday Trans. 1 77, 2907 ( 1981). 6. Singh, H. N., Saleem, S. M., Singh, R. P., and Birdi, K. S., J. Phys. Chem. 84, 2191 (1980). 7. Evans,D. F., Chen, S. H., Schriver,G. W., and Arnett, E. M., J. Amer. Chem. Soc. 103, 481 (1981). 8. Evans,D. F., and Wightman, P. J., J. Colloid Interface Sci. 86, 515 (1982). 9. Evans, D. F., and Ninham, B. W., J. Phys. Chem. 87, 5025 (1983). 10. Friberg, S. E., and Liang, Y-C., in "Microemulsions: Structure and Dynamics" (S. E. Friberg and P. Bothorel, Eds.), Chap. 3. CRC Press, Boca Raton, FL, 1987. 11. Fletcher, P. D. I., Galal, M. F., and Robinson, B. H., J. Chem. Soc. Faraday Trans. 1 80, 3307 (1984).
CONCLUSIONS
In this work it has been shown that surfactant solutions with the nonaqueous solvents FM, NMF, and D M F behave differently in some respects and no close resemblances to Journal of Colloid and Interface Science, Vol. 142,No. 1, March 1, 1991
NMR OF SDS IN NONAQUEOUS SOLUTIONS 12. Rico, I., and Lattes, A., Z Colloid Interface Sci. 102, 285 (1984). 13. Rico, I., and Lattes, A., Nouv. J. Chim. 8, 429 (1984). 14. Samii, A. A., Savignac, A., Rico, I., and Lattes, A., Tetrahedron 41, 3683 (1985 ). 15. Almgren, M., Swarup, S., and Lofroth, J. E., Z Phys. Chem. 89, 4621 (1985). 16. Rico, I., and Lattes, A., J. Phys. Chem. 90, 5870 (1986). 17. Belmajdoub, A., E1Bayed, K., Brondeau, J., Canet, D., Rico, I., and Lattes, A., J. Phys. Chem. 92, 3569 (1988). 18. Das, K. P., Ceglie, A., and Lindman, B., J. Phys. Chem. 91, 2938 (1987). 19. Das, K. P., Ceglie, A., Monduzzi, M., Soderman, O., and Lindman, B., Prog. Colloid. Polym. Sci. 73, 167 (1987). 20. Lindman, B., Soderman, O., and Wennerstrom, H., in "Surfactant Solutions: New Methods of Investigation" (R. Zana, Ed.). Dekker, New York, 1987. 21. Chachaty, C., Prog. NMR Spectrosc. 19, 183 (1987). 22. Marchal, J. P., Caner, D., Neff, H., Rhobin-Lherbier, B., and Cases, J. M. J. Colloid Interface Sci. 99, 349 (1984). 23. Wennerstrom, H., Lindman, B., Soderman, O., Drakenberg, T., and Rosenholm, J. B. J. Amer. Chem. Soc. 101, 6860 (1979).
139
24. Heatly, F., J. Chem. Soc. Faraday Trans. 1 83, 2593 (1987). 25. Soderman, O., and Henriksson, U., J. Chem. Soc. Faraday Trans. 1 83, 1515 (1987). 26. Soderman, O., Walderhaug, H., and Lindman, B., aT. Phys. Chem. 89, 1795 (1985). 27. Lindman, B., and Stilbs, P., in "Microemulsions: Structure and Dynamics" (S. E. Friberg and P. Bothorel, Eds.), Chap. 5. CRC Press, Boca Raton, FL, 1987. 28. Soderman, O., Carlstrom, G., Olsson, U., and Wong, T. C., J. Chem. Soc. Faraday Trans. 1 84, 4475 (1988). 29. Monduzzi, M., Ceglie, A., Lindman, B., and Soderman, 0., J. ColloidlnterfaceSci. 136, 113 (1990). 30. Abragam, A., "The Principles of Nuclear Magnetism," Clarendon, Oxford, 1961. 31. Soderman, 0., J. Magn. Reson. 68, 296 (1986). 32. Davis, J. H., and Jeffrey, K. R., Chem. Phys. Lipids 26, 87 (1977). 33. Wennerstrom, H., Lindblom, G., and Lindman, B., Chem. Scr. 6, 97 (1974). 34. Halle, B., and Wennerstrom, H., J. Chem. Phys. 75, 1928 (1981). 35. Stilbs, P., Rapacki K., and Lindman, B., J. Colloid Interface Sci. 95, 583 (1983). 36. Ceglie, A., Das, K. P., and Lindman, B., ,L Colloid InterfaceSci. 115, 115 (1987).
Journal of Colloid and Interface Science, Vol. 142, No. 1, March 1, 1991
M. MONDUZZI,'~'z~ AND O. S O D E R M A N §
* Dipartimento di Chimica, Universitd di Bari, 70126 Bari, Italy; tDipartimento di Scienze Chimiche, Universitd di Cagliari, 09124 Cagliari, Italy, and §Physical Chemistry 1, Chemical Centre of Lund, S-221 O0 Lund, Sweden
Received October 27, 1989; revised July 2, 1990 A multifield 2H and ~3C NMR relaxation study was carried out on some two-, three-, and fourcomponent systems of sodium dodecyl sulphate in nonaqueous solvents such as formamide, N-methyl formamide, or N,N-dimethyl formamide. Pentanol or octanol was used as cosurfactant, while p-xylene was added to obtain nonaqueous microemulsions. Relaxation data were mainly analyzed in terms of a "two-step" motional model in order t o obtain details on the structure and dynamics of the systems. The analysis of the data demonstrated some peculiar features of these nonaqueous systems, and analogies and differences with respect to the corresponding aqueous systems were investigated. It was shown that the degree of organization of SDS in the nonaqueous solvents is generally rather low. Among the results the effect of the addition ofoctanol as cosurfactant to the two-component systems should be mentioned. In the case of formamide systems, this alcohol seems to induce the formation of an organized solution, although the occurrence of any closed domain cannot be proved. A totally different behavior is observed in N-methyl formamide and N,N-dimethyl formamide. In these solvents, octanol seems to act as a structure breaker, thus leading to largely structureless solutions. © 1991AcademicPress,Inc. INTRODUCTION S u r f a c t a n t aggregates in a q u e o u s s o l u t i o n have b e e n widely investigated b y m e a n s o f m a n y different techniques. In the last d e c a d e the interest has also focused on the s t u d y o f surfactant self-association in n o n a q u e o u s syst e m s a l t h o u g h aggregation p h e n o m e n a o f s u r factants in s o m e organic p o l a r solvents have been p r e v i o u s l y o b s e r v e d ( 1- 13 ). M o r e o v e r , n o n a q u e o u s m i c r o e m u l s i o n s have b e e n used as r e a c t i o n m e d i a ( 1 4 ) . These facts h e l p e d to rise the interest in a n d the d e m a n d for a deeper k n o w l e d g e o f these n o n a q u e o u s systems. A t present, a l t h o u g h the phase d i a g r a m s o f several n o n a q u e o u s systems have b e e n determ i n e d , very little is k n o w n c o n c e r n i n g m i c r o structure, shape, a n d size o f the aggregates a n d flexibility o f the interfaces. C o n t r a d i c t o r y results have been o b t a i n e d b y different experim e n t a l t e c h n i q u e s even o n the m o s t investiTo whom correspondence should be addressed.
gated systems. F o r instance, the o c c u r r e n c e o f micellar aggregates o f s o d i u m dodecyl sulphate ( S D S ) in f o r m a m i d e ( F M ) was h y p o t h e s i z e d in Ref. ( 6 ) on the basis o f c o n d u c t a n c e m e a surements. L a t e r it was shown t h a t s o m e k i n d o f " s o l u t e - s o l v e n t particles" i n s t e a d o f true micelles o c c u r in this system ( 15 ). H o w e v e r , the o c c u r r e n c e o f SDS micelles in F M was indicated at higher t e m p e r a t u r e s ( T a b o v e 5 5 °C) ( 1 6 ) . Likewise in the c e t y l t r i m e t h y l a m m o n i u m b r o m i d e - F M system the o c c u r r e n c e o f spherical micelles at 6 0 ° C was a s c e r t a i n e d b y m e a n s o f a 1H, ~3C, a n d 14N multifield N M R r e l a x a t i o n study ( 1 7 ) . R e c e n t l y a r a t h e r extensive N M R self-diffusion s t u d y ( 1 8 ) , carried o u t at r o o m t e m p e r a t u r e o n s o m e two-, three-, a n d f o u r - c o m p o n e n t SDS systems with F M , N - m e t h y l formamide (NMF), and N, N-dimethyl forma m i d e ( D M F ) as solvents, d i d not indicate a n y appreciable confinement of any component in closed d o m a i n s , or the presence o f interfaces
129 0021-9797/91 $3.00 Journal of Colloid and Interface Science, Vol. 142, No. 1, March 1, 1991
Copyright © 1991 by Academic Press, [nc. All rights of reproduction in any form reserved.
130
CEGLIE, MONDUZZI,
as found for the aqueous systems. Some preliminary 2H relaxation data (19) showed that SDS does not give any appreciable micellization in FM at 26°C, in agreement with selfdiffusion data ( 18 ) and previous findings ( 15, 16). However, the addition of octanol to this system seems to induce some kind of organization. The same trend is found in the fourcomponent S D S - F M - o c t a n o l - p - x y l e n e microemulsion; however, relaxation data were not sufficient to gather details on molecular dynamics. Therefore we have undertaken a systematic study of these nonaqueous systems by means of 2H and 13C N M R relaxation to clarify some aspects of the aggregation phenomena, if any, at room temperature. Multifield 2H and 13C relaxation studies have been extensively used to investigate molecular motions in surfactant solutions (2026). In many cases the rationalization of the spin-lattice and spin-spin relaxation data in micellar surfactant-water systems was made by the use of a motional model, the so-called "two-step model" (23), on the basis of which one considers two kinds of motions: a fast local motion, slightly anisotropic, due to the reorientation of the methylene groups within the alkyl chain of the surfactant, and a slower motion, isotropic and superimposed on the former, due to the tumbling of the whole aggregate a n d / o r to the diffusion of the surfactant monomers over the aggregate surface. This model was proved to apply also in the case of several aqueous microemulsions (20, 27). From the analysis of N M R relaxation data we want to draw a profile of structure and dynamics of these waterless systems in order to identify analogies with and differences from the corresponding aqueous systems (19, 28, 29). THEORETICAL
BACKGROUND
The 13C spin-lattice relaxation rates of methyl and methylene carbons are mainly due to the dipole-dipole interaction between carJournal of Colloid and Interface Science, Vol. 142, No. 1, March 1, 1991
AND SODERMAN
bons and their directly bound protons. The expression for 13C R 1 is given by (30)
L 4S r . + 3J(wc) + 6J(wH + wc)l.
[ll
For a nucleus with spin I = 1, like deute-
rium, the spin-lattice (R1) and spin-spin (R2) relaxation rates are (30) 37r 2 R, = ~ - - X2[2J(wD) + 8J(2c0v)]
[2]
37r2 R2 = ~ x213J(0) + 5J(WD) + 2J(2wD)]. [31 In these equations Ni is the number of protons directly bound to the carbon; ~zo is the permeability of vacuum; ~/r~ and 7c are the magnetogyric ratios of proton and carbon; rcn is the C - H bond length (set equal to 1.09 A, the value normally used for a sp3-hybridized carbon: it has been suggested that a value of 1.11 A ( 31 ) should be used; however, negligible differences are obtained for the parameters calculated with these two values of rcH); J(w) are the various reduced spectral density functions; and Wc, wn, and wD are the Larmor frequencies of carbon, proton, and deuterium at a given magnetic field strength. In Eqs. [ 2 ] and [3], x is the quadrupolar coupling constant. Please note that the asymmetry parameter of the electric field gradient tensor for the C - D group is set equal to zero (32). By introducing in the rationalization of the relaxation data the two-step model, which assumes a slow isotropic motion superimposed on a fast, slightly anisotropic, motion and by assuming that the "extreme narrowing conditions" apply to the fast motion and furthermore that the slow motion can be described by a single exponential correlation function, we can write the reduced spectral density function as (33, 34) J(w) = ( 1 -- S2)2rcr+ S 2
2r~
1 + (oOr~c) 2 "
[4]
NMR OF SDS IN NONAQUEOUS SOLUTIONS S is the order parameter defined as S = (½)(3 cos2~9 1 ) f , where the average is taken over a time that is long enough to average the anisotropic fast local motion but much shorter than the time characteristic of the slow motion. 0 represents the angle between the C - H or C - D vector and the local director, taken to be normal to the surface of the aggregate. ~-~ and r~ are the correlation times for the fast and slow motions, respectively. -
-
EXPERIMENTAL Materials. Sodium dodecyl sulphate (99%,
biochemical grade), 1-pentanol (98 % ) (PEN), 1-octanol ( 9 9 % ) ( O C T ) , formamide (analaR), N-methyl formamide (analaR), N,N-dimethyl formamide (analaR), and pxylene (98.5%) ( X Y L ) were all from BDH, England. All the reagents were used as received without further purification, a,a-2H2-SDS (DSDS) was prepared following a procedure previously described (29). Methods. N M R 13C relaxation measurements were run at 1.4, 2.35, 4.7, and 8.47 T on a Jeol-FX-60, on a Bruker MSL 100, on a Varian XL-200, and on a Nicolet NTC-360 spectrometer, respectively. 2H N M R relaxation measurements were performed at 4.7 T on the Varian XL-200 spectrometer. In the 13C experiments ~H decoupling was carried out by means of square wave modulation of the decoupler carrier centered in the proton frequency range. At the highest magnetic field strength a bilevel decoupling sequence was used in order to avoid overheating phenomena. The temperature in all measurements was 26.0 + 0.5°C. The partially relaxed spectra were obtained by means of the usual "inversion recovery" sequence. The spin-lattice relaxation rates were calculated using a three-parameter nonlinear fitting to 14-20 experimental points. The spin-spin relaxation rates for 2H were deduced from the bandwidths of the 2H N M R signals taken at half-height after suitable correction for the magnetic field inhomogeneity. Computational details. With reference to
131
the two-step model we assumed that the acarbon of the surfactant was the most sensitive to the slow motion as it is closest to the nonpolar-polar interface. Then we chose a value ofr~ and from the experimental 13C Rl values of the a-carbon obtained at two different magnetic field strengths we calculated analytically the values of r f and S. These values of r~c, z~r, and S were used to calculate the relation 2HR1 Rcalc = 2HR2 2J(WD) + 8J(2wD) = 3 ) ( 0 ) + 5](~D) + 2J(2~0D),
[5]
which is the ratio between Eqs. [2] and [3] after the parameter X2 is eliminated. The function J(a~) is given by Eq. [4]. The value of Rc,lc so obtained was compared with that obtained from the experimental ratio Robs = [R1/R2] for 2H. The value ofr~ which minimizes the quantity ERR =
(Robs - RcaIc) 2 (Robs) 2
[6]
was then chosen as the best value ofr~ for the whole aggregate, r f and S values for all resolved carbons of the surfactant were then calculated. When 13C R1 values at more than two magnetic field strengths were available, these other data were included in the minimization procedure and the minimized quantity becomes ERRa-= ERR + ~
(Rlobs
-- Rlcalc)
~b~
2
, [7]
K,I
where K and I indicate the magnetic field strength and the carbon, respectively. RESULTS AND DISCUSSION In Table I we report the 2H relaxation data together with the composition (wt%) of all the examined samples. In the same table the 2H relaxation rates obtained at 6.0 and 8.47 T for some three- and four-component systems with FM (from Ref. (19)) are also reported while the 2H data obtained for the corresponding Journal of Colloid and Interface Science, Vol. 142, No. 1, March 1, 1991
132
CEGLIE, MONDUZZI, AND SODERMAN TABLE I 2H Relaxation Data at 26°C for Samples Containing DSDS 4.7 T
Systemand composition(wt%) SDS-FM (1-99) (4-96) (6.5-93.5) SDS-FM-alcohol (15-55-30) PEN OCT SDS-FM-alcohoI-XYL (16-40-32-12) PEN OCT SDS-NMF (2-98) (8-92) 15-85) SDS-NMF-alcohol (15-55-30) PEN OCT SDS-NMF-alcohoI-XYL (16-40-32-12) PEN OCT SDS-DMF (2-98) (5-95) (8-92) SDS-DMF-alcohol (11-67-22) PEN OCT SDS-D20 d (5-95) (10-90) (20-80) SDS-H20-PEN a (20-43.6-36.4) SDS-H20-alcohol-XYL a (17.5-35-35-12.5) PEN OCT
RIa (s-I)
R2b(s-~)
16.7 +_ 0.3 18.3 +_ 0.1 20.6 +_ 0.2
19.0 20.7 24.7
0.88 +_ 0.07 0.88 +_ 0.07 0.83 +_ 0.05
36.1 +_ 0.3 37.9 +_ 0.3
48.5 79.4
0.74 +_ 0.02 0.48 +_ 0.01
36.0 +_ 0.3 39,5 +_ 1.1
49.8 87.0
0.72 +_ 0.02 0.42 +_ 0.01
11.1 +_ 0.3 12.3 _+ 0.1 15.6 +_ 0.1
14.6 16.0 19.0
0.76 +_ 0.08 0.75 +_ 0.08 0.82 _+ 0.07
19.5 _+ 0.1 24.5 +_ 0.2
23.3 25.4
0.84 +_ 0.06 0.96 +_ 0,06
23.2 +_ 0.2 28.7 +- 0.3
24.1 28.8
0.96 + 0,06 0.99 +_ 0,06
6.9 +_ 0.1 7.8 +_ 0.1 8.8 +_ 0.1
9.1 10.1 10.7
0.76 +_ 0,13 0.77 +_ 0.12 0.82 +_ 0.12
12.8 +_ 0.1 14.0 +_ 0.1
17.3 16.0
0.74 +_ 0.07 0.87 _+ 0.09
28.4 +_ 0.7 29.2 _+ 0.2 31.0 +- 0.2
42.9 48.0 51.0
0.66 +_ 0.03 0.61 _+ 0.02 0.61 +- 0.02
26.0 +_ 0.2
56.8
0.46 +_ 0.01
27.0 +_ 0.1 26.2 +_ 0.2
54.6 182.8
Ro~C
0.49 +_ 0.01 0.14 +_ 0.002
6T
SDS-FM-OCT f (15-55-30) (10-40-50) SDS-FM-OCT-XYL y (13,3-30-26.8-29.9) (13,4-45-26.7-14.9)
8.47 T
R~~(s-')
Rf (s-~)
Ro~~
R~~(s~)
32.9 +_ 0.9 32.2 +_ 1.0
62.5 +_ 3.9 55.6 +_ 6.2
0.53 +_ 0.04 0.58 +_ 0.07
27.9 +_ 0.2 27.2 +_ 0.4
29.9 +_ 0.6 26.3 _+ 0.7
79.4 +_ 5.0 66.7 _+ 4.4
0.38 +_ 0.03 0.39 _+ 0.03
26.1 +_ 0.2 25.6 + 0.2
a The reported errors in R1 represent the error of the three-parameter nonlinear fitting. b R2 data are obtained from 2H N M R bandwidth taken at half-height in a spectrum recorded with a pulse angle of 90 °. A Lorentzian line fitting of the experimental data points confirmed the Lorentzian shape of the 2H NMR signals to within an error of +_0.5 Hz, which corresponds to an error of _+).6 s-1 in R2. The correction for the inhomogeneity contribution was made on the basis of the values of Rl and R2 of a D20 reference sample measured before and after recording each sample. c The error in R2 corresponds to the propagated error resulting from the errors in R1 and R2. d From Ref. (29). e From Carr-Purcell-Meiboom-GiU experiments. f F r o m Ref. (19). Journal of Colloid and lnterface Science, Vol. 142,No. 1, March 1, 1991
NMR OF SDS IN NONAQUEOUS SOLUTIONS 136R1
t
SDS-
S-1
7 -93
2.0
,i
SDS-NMF
FM
15
%
-
85
,) ~ ,~ (, ,b (,
,)
,)
o
Z
:
#
-{
####~Q
#l
# ~01 A i ~A| #* •
hO
I
|
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123456789101112
1 23456789101112
SDS
carbons
FIG. I. Experimental ~3CR~ for the carbons of SDS in the two-component systems. Frequency: (O) 1.4 T, (11) 4.7 T, (A) 8.47 T. aqueous systems are given for comparison (from Ref. (29)). The ~3C spin-lattice relaxation rates R~ are shown in Fig. 1 for the twoc o m p o n e n t systems, in Fig. 2 for the threec o m p o n e n t systems, and in Fig. 3 for the fourc o m p o n e n t microemulsions with pentanol as cosurfactant.
~3C R
Some considerations arise from a first observation of the experimental results: (a) In the micellar S D S - H 2 0 system the 2H R~ and R2 values and, in particular, their ratios, Robs'S, were found to be almost constant with increasing SDS concentration above CMC. This fact is indicative of the occurrence of an almost constant ratio between micellized and free surfactant molecules. The values obtained for Robs above C M C are around 0.6, while below CMC the value is almost equal to unity (Robs = 0.93 _+ 0.15). Thus a value of Rob s significantly less than unity is indicative of the presence of surfactant aggregation. In the nonaqueous systems the values of Robs are around 0.8, thus indicating that the surfactant molecules occur to a large extent as monomers. A m o n g the three- and four-component systems, only the FM systems containing octanol as cosurfactant show Rob s values significantly lower than unity. In the aqueous threeand four-component systems the Robs ratios are always lower than 0.6, in particular, in the octanol microemulsion, where a value of 0.14 was found.
SDS-FM-PEN
S-1
15
- 55
- 30
133
SDS-NMF-PEN %
15
-
55
- 30
%
2.0
T A I
I
I
1.0
Aeee
•
1
¢
1
•JA!
[] •
SDS 2 34
56
El •
[]
•
!
• D
PEN 7 891011
1 2 3 4
SDS 234
5 6 7 8 91011
D D
PEN 1 2 34
carbons
FIG. 2. Experimental ~3C R~ for the carbons of SDS and PEN in the three-component systems with pentanol as cosurfactant. Frequency: ([]) 2.35 T, (i) 4.7 T, (A) 8.47 T. C~2signal of SDS is obscured by C5 of PEN. Journal of Colloid and Interface Science, Vol, 142, No. 1, March 1, 1991
134
CEGLIE, MONDUZZI, AND SODERMAN
13C Rlt
SDS- FM- PEN- p-XY L 16-40-
S D S - N M F - PEN-p-XYL
12
-
16
-
40-
32
-
12
t
2.0
1.0
32
+
":::t
t
• 1
Iiti 1
!
D
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| ~DS
PEN
1 234567
891011
PEN
SDS
I 234567891011
1 234
1 234
carbons FIG. 3. Experimental ~3C R~ for the carbons of SDS and PEN in the four-component systems with pentanol as cosurfactant and p-xylene as oil. Frequency: ([3) 2.35 T, (am)4.7 T, (A) 8.47 T. C~z signal of SDS is obscured by C5 of PEN and by the -CH3 groups of XYL
( b ) I n Figs. 1, 2, a n d 3 it can be seen t h a t t h e f r e q u e n c y d e p e n d e n c e o f the 13C R~'s in the various systems is generally larger for the a - c a r b o n o f SDS t h a n for the o t h e r carbons. T h i s is p a r t i c u l a r l y e v i d e n t in Figs. 4 A - 4 C ,
A
w h e r e the ratios b e t w e e n the 13C R~'s o b t a i n e d at t w o different frequencies for each c a r b o n in the v a r i o u s systems are reported. In the s a m e figure we also report, for c o m p a r i s o n , the ratios o b t a i n e d in t h e c o r r e s p o n d i n g a q u e o u s sys-
13 2 . 3 5 n 1
B
Rp-47
R~138 R~.47
~¢r
¢r ¢r ¢r ¢r'~'~ •
•
SDS
123456789101112
SDS
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C
rosa
m
~
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~
PEN
SDS
PEN
1234
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carbons FIG. 4. Ratio between 13C R 1 'S for each carbon of SDS at two different frequencies: (A) SDS in the twocomponent systems, (R~h.4/(Rl)8.47; (B) SDS and PEN in the three-component systems, (RI)2.35/(R1)8.47; (C) SDS and PEN in the four-component systems, (Rx)2.35/(Rt)8.47. 13Cdata for the aqueous systems is from Ref. (29). (~) Aqueous systems, (11) FM systems, ([]) NMF systems. Journal of Colloid and Interface Science, Vol. 142, No. 1, March 1, 1991
135
NMR OF SDS IN NONAQUEOUS SOLUTIONS
marked frequency dependence of 13CR l's only in the aqueous systems (Figs. 4B and 4C). On the basis of these considerations it is evident that important differences between aqueous and nonaqueous systems occur in the molecular organization and in the dynamic behavior. In the analysis of the experimental data it must be taken into account that FM, NMF, and DMF differ from water in many important physical properties such as viscosity and dielectric constant (see Table II). However, since a frequency dependence of 13C R~ is observed and 2H R1 differs from 2H Rz in several cases, at first we made use of the twostep model approximation. Following the procedure described under s r~, and S for Experimental we calculated rc, the various systems. These parameters are reported together with the values of ERR (Eq. [ 6 ] ) and ERRT (Eq. [ 7 ]) in Tables III and IV.
TABLE 1I Viscosities 0/) and Dielectric Constants (~) of the Solvents at 25°C Solvent
~/(cP)
~a
H20 FM NMF DMF
0.891 3.30 1.65 0.796
78.8 109.5 182.4 36.7
a From the "Handbook of Chemistry and Physics," CRC Press, Cleveland, OH, 1983-1984.
tems (from Ref. (29)). The trend of these ratios indicates that the frequency dependence of the 13C R~'s in the nonaqueous two-component systems is significantly lower than that observed in the SDS-water micellar system (Fig. 4A). The same trend, although much less pronounced, is observed in the four-component systems (Fig. 4C), while no significant differences from the corresponding aqueous system are observed in the three-component systems with pentanol as cosurfactant (Fig. 4B). The a-carbon of pentanol shows a
FM Systems In the case of the two-component system, the experimental ~3CRl'S of the SDS a-carbon are very well reproduced by the values of r g
TABLE III FM Systems SDS-FM z~ = 2.5 ns
SDS-FM-PEN r~ = 7.0 ns
~
SDS-FM-PEN-XYL ¢~ = 7.0 ns
~
T~r
Carbon
(ps)
S
(ps)
S
(ps)
S
1 2 3 4 5 6 7 8 9 10 11 12
18.1 32.4 28.9 22.7 31.0 29.3 26.2 29.3 22.7 14.1 10.7 4.3
0.141 0.094 0.130 0.124 0.099 0.104 0.114 0.104 0.124 0.085 0.072 0.037
25.2 34.0 35.5 21.1 32.3 27.8 27.8 27.8 21.1 14.0 9.8 .
0.127 0.082 0.121 0.135 0. l 17 0.133 0.133 0.133 0.135 0.059 0.043
26.1 32.6 29.7 32.6 24.0 24.8 24.8 24.0 19.7 11.3 8.1
0.109 0.051 0.134 0.051 0.126 0.123 0.123 0.126 0.084 0.051 0.043
ERR ERR~
0.80 × 10-3 0.160
. 0.75 × 10-~ 0.73
.
. 0.33 × 10-~ 0.289
N o t e . r~, r~, and S parameters were calculated with the two-step model of approximation for SDS carbons. Journal of Colloid and Interface Science. Vol. I42, No. 1, March 1, 1991
136
CEGLIE, MONDUZZI, AND SODERMAN TABLE IV NMF Systems SDS-NMF r~ = 2.5 ns
Carbon
1
2 3 4 5 6-8 9 10 11 12 ERR ERRT
SDS-NMF-PEN r~ = 5.4 ns
SDS-NMF-PEN-XYL r~ = 6,0 ns
r re (ps)
S
(ps)
S
(ps)
S
16.8 16.2 19.9 16.2 18.1 16.2 16.2 8.1 7.2 3.8
0.111 0.091 0.102 0.091 0.084 0.091 01091 0.064 0.023 0.016
19.0 16.2 25.9 17.3 21.1 21.1 17.3 10.3 7.3
0.111 0.130 0.087 0.091 0.089 0.089 0.091 0.051 0.030
18.8 30.2 28.6 20.2 22.0 22.0 20.2 9.2 7.0
0.130 0.026 0.097 0.076 0.089 0.089 0.076 0.064 0.034
0.1 × 10 3 0.17 X 10-1
0.72 × 10-1 0.40
0.190 1.45
Note. T~, r~, and S parameters were calculated with the two-step model of approximation for SDS carbons.
= 2.5 ns, r~ = 18.1 ps, a n d S = 0.141 reported observed 13C R1 and 2H R1 and R2 relaxation in Table III, ~whereas 2H relaxation rates data. In fact motions should be rather complex slightly higher than the experimental values since they arise from rotations around carbonare calculated when X = 185 kHz (29) is in- carbon bonds, torsions, and rotations of the troduced in Eqs. [2 ] and [3 ]. A slow-motion whole molecule around its long axis. Moreover correlation time rg = 2.5 ns seems to be an motions exceeding the extreme narrowing unreliable value for interpreting the micellar limit can be expected either in the case where tumbling in a solvent such as FM. This solvent only SDS monomers occur in a viscous solvent has a viscosity which is three times higher than such as FM or in the case where small SDS that of water at room temperature; therefore, dimers, trimers, etc., a n d / o r some kind of slow-motion correlation times of ca. 10-8 s are molecular solute-solvent complexes are presexpected if SDS micelles of the same size as ent. It should be noted that the occurrence of those found in aqueous solution were present a molecular complex could account for the in FM solution. Moreover, the observation observed conductance variation (6) and could that 2H R1 depends rather strongly on the con- also be in agreement with the observation that centration of SDS implies that a substantial the self-diffusion coefficient of FM increases fraction of SDS molecules is present as mono- while that of SDS is almost constant as the mers. These results are in agreement with the : FM concentration increases i n the tv~o~com-. conclusions d r a w n self-d~ffusiofi,;mea~; " P o n e i a f s D S ' F M gygtem (i8i:. ' ~ ...... In conclusion, although in this case the twosurements and with our preliminary relaxation data (18, 19) as well as with the results ob- step model should not appropriately describe tained by fluorescence quenching in (15), the motional behavior of SDS molecules in which indicate the occurrence of a molecular FM solution, it can be considered as a qualitative description of the local motion (cf. r f) dispersed solution in the SDS-FM system. Although in this system no well-defined ag- and the degree of freedom (cf. S parameter) gregation form occurs, a single isotropic cor- of the various carbons and of the tumbling of relation time is inadequate for describing the the whole SDS molecules in FM (cf. ~_s). Journal of Colloid and Interface Science, Vol. 142, No. 1, M a r c h 1, 1991
N M R OF SDS IN N O N A Q U E O U S SOLUTIONS
In the three- and four-component FM systems with pentanol as cosurfactant we observe a marked increase of the spin-lattice and especially of the spin-spin 2H relaxation rates and consequently a decrease of the ratio 2H R~/R2 (see Table I). The analysis of the ~3C and 2H relaxation data in terms of the twostep model gave the results reported in Table III. Generally the ~3C R~ data of the a-carbon can be fairly well reproduced by these parameters but they cannot reproduce the 2H R~ and R2 values when X = 185 kHz is used in the calculations. As for the three-component system S D S F M - O C T , unfortunately the overlapping of most of the ~3C N M R signals of SDS with those ofoctanol prevented us from performing a detailed relaxation analysis. However, in this case, 2H relaxation data for the a-carbon of SDS are available at three different frequencies (19). It should be noted that the 2H R~ for the S D S - F M - O C T system does not depend on SDS concentration at 6.0 and 8.47 T (see Table I). Through the use of the two-step model the frequency dependence of the 2H relaxation rates can be accounted for by the values r~ = 6 ns, rcf = 35 ps, and S = 0.18, assuming X = 185 kHz (29). The results are shown in Fig. 5, where it can be seen that a fairly good agreement between the calculated and experimental 2H R~ and R2 values is obtained. In the case of the microemulsion with oc-
137
tanol, a further decrease of the ratio 2H R~/ R 2 is observed; however, the available experimental data give evidence of a dynamic behavior very like that found in the S D S - F M O C T system. The addition of the oil seems to induce a slight decrease in the rate of the slow motions. From self-diffusion coefficients of FM in these systems it can be inferred that FM is not extensively confined into closed domains. In particular, it should be mentioned that in the corresponding aqueous microemulsions the relaxation and self-diffusion data indicate the occurrence of a bicontinuous structure (Robs = 0.49) when pentanol is used as cosurfactant and of a water-in-oil droplet structure (Robs = 0.14) for the microemulsion with octanol as cosurfactant ( 18, 29, 35, 36 ). However, the fact that FM occurs in a continuous domain does not exclude the possibility that the SDS molecules are organized in some sort of aggregate, as shown by the 2H relaxation data. It should be noted that self-diffusion coefficients are significantly affected by the presence of a small fraction of nonassociated molecules, since Dobs = pDmon + ( 1 - p)Dagg (p is the fraction of molecules present as m o n o mers while 1 - p is the fraction of aggregated molecules) and Dmon ~ Dagg. On the other hand, relaxation measurements are significantly influenced by the presence of a small a m o u n t of aggregated molecules, since in this case Rlobs = pRlmoo + ( 1 - P)Rlagg and R1agg Rlmon.
S-1 2H R2
S D S - FM - O C T
lOO
50
Iog~ ?.5
8.0
FIG. 5. S D S - F M - O C T
8.5
9.0
9.5
In these systems the relaxation measurements, interpreted in terms of the two-step model, revealed significant differences arising from the use of different alcohols: octanol, unlike pentanol, seems to induce the formation of some kind of well-defined interface. In support of this the goodness of the results obtained in the case of the S D S - F M - O C T system by the use of the two-step model should be noted.
( 1 5 - 5 5 - 3 0 wt%) (O) 2H
N M F Systems (R l) expand (•) 2H (R2)expexperimental relaxation rates. Continuous lines: two-step model ZH (RI)~lc and (Rz)calc The experimental data for the two-comas a function oflog(oa)with X = 185 kHz, r~ = 6 ns, ~-~ = 35 ps, S= 0.18. ponent systems with N M F as solvent seem to Journal of Colloid and Interface Science, Vol. 142,No. 1, March i, 1991
138
CEGLIE, MONDUZZI, AND SODERMAN
indicate the occurrence of a structure and of a dynamic behavior rather similar to those found in the FM systems. On the contrary the 2H data, reported in Table I, show that the addition of the third and the fourth component induces an increase of the ratio 2H R l / R2 either in the case of pentanol or in the case of octanol. What is interesting is the observation that the use of octanol as cosurfactant seems to produce almost molecular-disperse solutions (cf. Table I, R1 = R2). The use of the two-step model led to the estimates of the parameters reported in Table IV. The set of parameters calculated for the two-component system can reproduce either the ~3C or the 2H relaxation rates rather accurately. Worse agreement is obtained in the case of the three- and four-component systems. The same considerations as those reported above for the FM systems, except those containing octanol, can be extended to N M F systems. In conclusion the N M R relaxation data seem to confirm the absence of any aggregated form of the surfactant molecules in N M F systems.
the corresponding aqueous systems have been displayed. One of the most interesting features is the effect of the addition of octanol to the systems with N M F and DMF. In these solvents, octanol seems to induce a complete breaking up of every kind of aggregation, unlike what has been observed in FM, where the occurrence of some sort of organized interface has been ascertained. The use of the two-step model is strictly correct only if some kind of organization is proved to occur to a certain extent, while in other cases the method represents only a qualitative approach to interpret molecular dynamics. Finally it should be noted that the analysis of the 2H relaxation rates seems to be very useful in discriminating between organized and molecular-dispersed solutions. ACKNOWLEDGMENTS
We are gratefulfor helpful discussionswith Bjorn Lindman. Financial support by the Swedish National Science Research Council, by the Swedish Board of Technical Development, and by the Italian National Council of Research (CNR), Comitato Tecnologico,is acknowledged. REFERENCES
DMF Systems
1. Mittal, K. L., and Fendler, E. J. (Eds.), in "Solution Behaviour of Surfactants," Vol. 2, part III, pp. 743-
The use of D M F as solvent should yield very different systems since both viscosity and dielectric constant are m u c h smaller than those of water, FM, and NMF. Only preliminary data are available for these systems; however, the 2H data, reported in Table I, suggest that the structure and the dynamic behavior should be very m u c h like those found in the N M F systems. Moreover, it should be pointed out that the further decrease of 2H relaxation rates in the D M F systems is clearly related to the decrease of the viscosity of this solvent as compared to that of F M and NMF.
886. 2. Reinsborough,V. C., and Bloom, H., Aust. Y. Chem. 20, 2583 (1967). 3. Reinsborough,V. C., and Bloom, H., Aust. J. Chem. 21, 1525 (1968). 4. Reinsborough,V. C., and Bloom, H., Aust. J. Chem. 22, 519 (1969). 5. Ionescu,L. G., and Fung, D. S., J. Chem. Soe. Faraday Trans. 1 77, 2907 ( 1981). 6. Singh, H. N., Saleem, S. M., Singh, R. P., and Birdi, K. S., J. Phys. Chem. 84, 2191 (1980). 7. Evans,D. F., Chen, S. H., Schriver,G. W., and Arnett, E. M., J. Amer. Chem. Soc. 103, 481 (1981). 8. Evans,D. F., and Wightman, P. J., J. Colloid Interface Sci. 86, 515 (1982). 9. Evans, D. F., and Ninham, B. W., J. Phys. Chem. 87, 5025 (1983). 10. Friberg, S. E., and Liang, Y-C., in "Microemulsions: Structure and Dynamics" (S. E. Friberg and P. Bothorel, Eds.), Chap. 3. CRC Press, Boca Raton, FL, 1987. 11. Fletcher, P. D. I., Galal, M. F., and Robinson, B. H., J. Chem. Soc. Faraday Trans. 1 80, 3307 (1984).
CONCLUSIONS
In this work it has been shown that surfactant solutions with the nonaqueous solvents FM, NMF, and D M F behave differently in some respects and no close resemblances to Journal of Colloid and Interface Science, Vol. 142,No. 1, March 1, 1991
NMR OF SDS IN NONAQUEOUS SOLUTIONS 12. Rico, I., and Lattes, A., Z Colloid Interface Sci. 102, 285 (1984). 13. Rico, I., and Lattes, A., Nouv. J. Chim. 8, 429 (1984). 14. Samii, A. A., Savignac, A., Rico, I., and Lattes, A., Tetrahedron 41, 3683 (1985 ). 15. Almgren, M., Swarup, S., and Lofroth, J. E., Z Phys. Chem. 89, 4621 (1985). 16. Rico, I., and Lattes, A., J. Phys. Chem. 90, 5870 (1986). 17. Belmajdoub, A., E1Bayed, K., Brondeau, J., Canet, D., Rico, I., and Lattes, A., J. Phys. Chem. 92, 3569 (1988). 18. Das, K. P., Ceglie, A., and Lindman, B., J. Phys. Chem. 91, 2938 (1987). 19. Das, K. P., Ceglie, A., Monduzzi, M., Soderman, O., and Lindman, B., Prog. Colloid. Polym. Sci. 73, 167 (1987). 20. Lindman, B., Soderman, O., and Wennerstrom, H., in "Surfactant Solutions: New Methods of Investigation" (R. Zana, Ed.). Dekker, New York, 1987. 21. Chachaty, C., Prog. NMR Spectrosc. 19, 183 (1987). 22. Marchal, J. P., Caner, D., Neff, H., Rhobin-Lherbier, B., and Cases, J. M. J. Colloid Interface Sci. 99, 349 (1984). 23. Wennerstrom, H., Lindman, B., Soderman, O., Drakenberg, T., and Rosenholm, J. B. J. Amer. Chem. Soc. 101, 6860 (1979).
139
24. Heatly, F., J. Chem. Soc. Faraday Trans. 1 83, 2593 (1987). 25. Soderman, O., and Henriksson, U., J. Chem. Soc. Faraday Trans. 1 83, 1515 (1987). 26. Soderman, O., Walderhaug, H., and Lindman, B., aT. Phys. Chem. 89, 1795 (1985). 27. Lindman, B., and Stilbs, P., in "Microemulsions: Structure and Dynamics" (S. E. Friberg and P. Bothorel, Eds.), Chap. 5. CRC Press, Boca Raton, FL, 1987. 28. Soderman, O., Carlstrom, G., Olsson, U., and Wong, T. C., J. Chem. Soc. Faraday Trans. 1 84, 4475 (1988). 29. Monduzzi, M., Ceglie, A., Lindman, B., and Soderman, 0., J. ColloidlnterfaceSci. 136, 113 (1990). 30. Abragam, A., "The Principles of Nuclear Magnetism," Clarendon, Oxford, 1961. 31. Soderman, 0., J. Magn. Reson. 68, 296 (1986). 32. Davis, J. H., and Jeffrey, K. R., Chem. Phys. Lipids 26, 87 (1977). 33. Wennerstrom, H., Lindblom, G., and Lindman, B., Chem. Scr. 6, 97 (1974). 34. Halle, B., and Wennerstrom, H., J. Chem. Phys. 75, 1928 (1981). 35. Stilbs, P., Rapacki K., and Lindman, B., J. Colloid Interface Sci. 95, 583 (1983). 36. Ceglie, A., Das, K. P., and Lindman, B., ,L Colloid InterfaceSci. 115, 115 (1987).
Journal of Colloid and Interface Science, Vol. 142, No. 1, March 1, 1991